The present invention relates to a sheet which is useful for various packings, bearings, thrust washers, etc., a process for producing the same, and the use of same. More particularly, the invention relates to a fluororesin sheet comprising an aromatic polyamide pulp and a fluororesin, as an essential component, the fluororesin sheet having excellent mechanical characteristics, friction and wear characteristics, heat resistance and chemical resistance, to a process for producing the same, and to a sliding material comprising the fluororesin sheet.
Because of its excellent friction and wear characteristics, heat resistance and chemical resistance, fluororesin is widely used as a sliding material for various packings, bearings, thrust washers, etc. However, fluororesin alone is liable to be deformed by external force, and the wear loss is high. Therefore, various reinforcing fillers have been formulated to correct these drawbacks.
For the above applications such as packing, thrust washer, etc., thin-sheet sliding material is required. However, it tends to be difficult to melt the fluororesin or dissolve it in a solvent. Therefore, it is impossible to use the sheet forming processes such as casting, blow moulding, etc., which are normally used in thermoplastic resins such as nylon, polyethylene, etc.
Accordingly, what tends to be used is a so-called skiving process comprising the subjecting of a cylindrical block to compression moulding and heating and skiving the resulting sheet to obtain a thin sheet. However, this process gives inferior productivity and, therefore, an economical process for producing a sheet article is needed which also provides high productivity.
Characteristics such as hardness, wear performance, etc., can be improved by further adding fibrous fillers (e.g., glass fiber, etc.) and particulate fillers (e.g., bronze powder, etc.), but the mechanical performance (e.g., tensile strength, etc.) cannot be improved because fluororesin's adhesion is inferior to that of the filler.
In order to improve this, the present inventors have suggested a resin composition for sliding material in which use is made of a highly-fibrillated aromatic polyamide pulp in combination with fluororesin powder (Japanese Patent Kokai (laid-open) No. 5-117476). Since the highly-fibrillated aromatic polyamide pulp is used as a reinforcement in this composition, the fluororesin is reinforced by physical interlocking of the aromatic polyamide pulp, which gives a strong reinforcing effect. However, also in this case a sufficiently strong reinforcing effect is obtained only in the block state.
In addition, a sheet produced from this block according to the skiving process has high anisotropy, and it is difficult to obtain sufficient tensile strength in all directions of the sheet surface. Therefore, materials having a tensile strength of not less than about 250 kg/cm.sup.2 in all directions of the sheet surface have never been obtained. The reason for this is considered to be that the fibers are mainly oriented in the direction perpendicular to the compressing direction at the time of press moulding of the block and scarcely in the compressing direction itself.
In Japanese Patent Kokoku No. 4-36181 is disclosed a process of depositing a tetrafluoroethylene resin polymer (PTFE) on a fiber sheet to give a sheet. In this process, however, chopped fibers made simply by cutting up long fibers are used as reinforcing fibers instead of the pulp and, therefore, a special mixer is required for the dispersion. In addition, a mixed solution of water and isopropanol is used as a dispersant, so there is a drainage treatment problem and normal paper-making processes using only water as the dispersant cannot be applied.
In Japanese Patent Kohyo No. 6-511029 is disclosed a process for producing a composite material which comprises bringing a mat obtained from a fiber material into contact with an aqueous dispersant of a fluoropolymer to precipitate the fluoropolymer onto the mat.
On the other hand, if a cut fiber is used without first making a fibrous mat, a problem is caused in that the rate of deposition of the fluororesin onto the cut fiber is too low.
As described above, there has never been a conventional technique for producing a sheet comprising an aromatic polyamide pulp as reinforcement and a fluororesin as matrix, with the sheet having sufficient tensile strength and the process having excellent productivity. Therefore, such a process is urgently needed.